1. Field of the Invention
The invention relates to the testing technology, and more particularly relates to methods for non-destructive testing with the help of penetrating substances, and may be used for revealing through and blind surface defects in materials and wares as well as for evaluating their dimensions.
2. Description of the Prior Art
Non-destructive testing methods, using penetrating substances, are attractive due to their simplicity and graphicity.
For revealing blind surface defects dye capillary methods are usually used, which methods comprise applying at the tested surface a coloured penetrant, typically this is a red paint, then removing an excess of the penetrant from this surface and applying at this surface a developer, typically this is a thin layer of white paint, which extracts the penetrant from defects and makes them "visible."
For revealing through defects leak detection methods are usually used, which methods use any test gas or liquid substance, filling up tested article under high pressure.
The method for detecting through and blind surface defects during tightness and strength testing is developed in Russia (Soviet Union No. 832411, published May 28, 1981), which method includes filling a pressure vessel with the indicator fluid, building up a pressure, holding this vessel for 30-120 minutes, applying an indicator paste at the external surface of this vessel and revealing through defects, and hereupon removing the indicator fluid from this vessel and from its internal surface, applying the indicator paste on this internal surface and revealing surface defects. But this method was designed for the limited set of test objects, namely for the closed vessels, made of strong materials. Moreover, this method has low sensitivity, especially in revealing through defects, which in turn depends on the thickness of the vessel wall, on the pressure inside of tested vessel, and on the viscosity of the indicator fluid.
Some years ago the new capillary-diffusion method for non-destructive testing of composite materials is developed in Russia (Proceedings of Moscow International on Composite Conference, November 14-16, pp. 773-777), which allows to reveal through and blind surface cracks, pores, porous zones and to evaluate sizes of defects. The excess of a penetrant evaporates from the tested surface, whereupon the penetrant begins to evaporate out of the defects. One puts on the tested surface the indicator coverings, which change their color when absorbing penetrant vapors. Color spots at the indicator coverings point to the presence of through or/and blind surface defects in the tested ware. However, this method does not allow to reveal defects having a depth less than 0.3 mm and a width less than 1 micrometer, which restricts the set of tested objects to a great extent and limits the sensitivity of testing.
For applying a penetrant at the tested surface one usually use a brush or an apparatus, which includes a reservoir for a penetrant and a sprayer, for example, aerosol bulb HELLING, for applying a liquid penetrant in form of small drops, spreading on the surface and forming a continuous layer of a liquid penetrant at the surface of tested object. Such apparatus does not contain a chamber for a specimen and does not allow applying a penetrant by means of adsorption and/or capillary condensation of penetrant vapours from the gas phase, which results in decreasing the sensitivity of testing and in limiting the variety of tested wares.
The capillary testing method was developed in Russia, which used the apparatus f or applying the penetrant from gas phase, which contains the sealed sample chamber with a blow-off valve, the joined reservoir for the penetrant fitted with a heater and a differential manometer, and a vapor collector joined to the valve (Soviet Union No. 1661632, published Jul. 7, 1991). The described known apparatus is designed for filling defects with kerosene penetrant by means of capillary condensation of its vapors. This known apparatus allows to fill only empty defects of the width less than 80 micrometers, but it does not allow to fill defects, which contain the condensed atmospheric moisture, as well as defects of the width more than 80 micrometers.
For making defects "visible" during testing by means of penetrating substances various types of indicator materials are used. For example, in dye capillary method the developer in the form of white paint is usually used, which paint is removed from the tested surface with special solvents, when testing is finished. This procedure demolishes traces of reveals defects.
Another type of indicator materials is the indicator covering, i.e. the material, comprising a base and a sensitive component, applied on this base, which keeps traces of revealed defects being removed as a unit from the tested surface.
There is the indicator material, developed in Russia, which comprises the base, made of fabric or filter paper, and the sensitive component, consisted of bisodium fluorescine salt (Soviet Union No. 667845, published Jun. 25, 1979). This indicator material is designed for registration weld defects by means of storing up moisture, penetrating through defects, followed by ultraviolet illumination, which initiates luminescence of wet spots. The above indicator material does not allow to reveal capillary defects, containing ammonia solutions, because of its low sensitivity.
There is another indicator material, developed also in Russia, which comprises the base, made of a cotton fabric, and the sensitive component, consisted of copper sulphate (Soviet Union No. 1610349, published Nov. 30, 1990). However, the chemical reactions, which proceed in this material in the presence of ammonia and which cause the change of the material color from light green to light blue, do not provide the adequate contrast of color change, which in turn restricts the sensitivity of the material. Moreover, the net structure of the indicator material base restricts its resolving power and does not allow to discern any color spots of sizes less than the fabric base cell dimensions, which also restricts the resolving power of the material. Besides that, this known indicator material is designed for leakage testing of closed articles, into which compressed ammonia is carried, which restricts a range of tested wares to a considerable extent.